1. Field of the Invention
The present invention relates, generally, to dispersion compensation systems and methods and, in preferred embodiments, to dispersion compensation using resonant cavities, and bandwidth and slope improvements in resonant dispersion filters.
2. Description of Related Art
Optical fiber systems have the potential for achieving extremely high communication rates, such as in the OC-192 and OC-768 high data rate systems. At these extremely high data rates, the modulation bandwidth is so large that even for an ideal source without chirp or phase noise, fiber dispersion, and particularly chromatic dispersion, becomes a critical issue. Fiber dispersion broadens the optical pulse and causes pulse spreading due to increased spectral width over long propagation distances. Therefore, fiber dispersion limits the distance that high data rate optical signals can be transmitted through optical fibers. The higher the data rate, the shorter the distance that optical signal can be transmitted without the need for dispersion compensation. Dispersion compensators are thus critical components for high data rate optical network systems.
Several methods for chromatic dispersion compensation (CDC) have been previously documented. In C. K. Madsen, G. Lenz, A. J. Bruce, et. al., “Multistage dispersion compensator using ring resonators”, Opt. Lett., vol. 24, no. 22, 1555 (1999), a demonstration was presented to compensate dispersion in periodic communication channels with two-stage ring resonators in planar waveguides. However, planar waveguides suffer from polarization-mode dependent dispersion and high sensitivity to temperature.
The group velocity dispersion (GVD) in optical fibers may be compensated by using a Gires-Tournois Interferometer (GTI). However, the useful depth of compensation in a single GTI is not adequate for long-distance, broadband applications. Furthermore, narrow bandwidth dispersions create problems in GTI-based resonant dispersion filters.
In C. K. Madsen and G. Lenz, “A multi-channel dispersion slope compensating optical allpass filter”, L. J. Cimini, L. J. Greenstein and A. A. M. Saleh, “Optical equalization to combat the effects of laser chirp and fiber dispersion”, J. Lightwave Tech., vol. 8, no. 5, 649 (1990), and F. Quellette, J.-F. Cliche, and S. Gagnon, “All-fiber devices for chromatic dispersion compensation based on chirped distributed resonant coupling”, J. Lightwave Tech. Vol. 12, no. 10, 1728 (1994), the use of GTIs to compensate for the chromatic dispersion in wavelength division multiplexing (WDM) systems was proposed. However, due to the narrow effective wavelength range in their system's channels, this method is rarely used in actual CDC systems, and no further investigation was reported.
In C. K. Madsen and G. Lenz, “A multi-channel dispersion slope compensating optical allpass filter”, a method of dispersion-slope compensation was mentioned by making the reflectivity of the lower reflection mirror in the GTI change with wavelength.
As a dispersion compensating device, a GTI is mainly used to obtain an ultra-short laser pulse, as was documented in R. Szipocs, A. Kohazi-Kis, S. Lako, et. al., “Negative dispersion mirrors for dispersion control in femtosecond lasers: chirped dielectric mirrors and multi-cavity Gires-Tournois interferometers”, Appl. Phys. B, vol. 70 [suppl.], s51-s57 (2000), B. Golubovic, R. R. Austin, M. K. Steiner-Shepard, et al., “Double Gires-Tournois interferometer negative-dispersion mirrors for use in tunable mode-locked lasers”, Opt. Lett., vol. 25, no. 4, 275 (2000), and R. R. Austin, B. Golubovic, “Multiple coupled Gires-Tournois interferometers for group-delay-dispersion control”, U.S. Pat. No. 6,081,379. Within the operating wavelength range (e.g. 700 nm˜900 nm) only one interval of the wavelength period structure appears. Therefore, the distance between the two reflecting mirrors is very short, about ½ of the center wavelength. In ultra-short laser pulse related research, in order to get the desired negative chromatic dispersion, a multi-cavity structure was proposed. Once again, though, these cavity lengths are on the order of a fraction of a wavelength.
The use of multiple reflections between two GTIs was adopted in an ultra-short laser pulse generating system in B. Golubovic, R. R. Austin, M. K. Steiner-Shepard et. al., “Double Gires-Tournois interferometer negative-dispersion mirrors for use in tunable mode-locked lasers”, Opt. Lett., vol. 25, no. 4, 275 (2000). Once again, though, these cavity lengths are on the order of a fraction of a wavelength.
The use of multiple reflections between two GTIs was also adopted to compensate dispersion slope in M. Jablonski, Y. Tanaka, H. Yaguchi, et. al., “Adjustable dispersion-slope compensator using entirely thin film coupled-cavity allpass filters in a multi-reflection parallel configuration”, OFC 2001. Because the working bandwidth is broad (about 3 nm), the GTIs used therein are thin film layers (about 10˜20 micron). However, this device cannot be used to be CDC device because the amplitude of negative dispersion is too small (<10 ps/nm).
The use of off-axis-illuminated multiple-coupled-cavity etalons has been described in P. Colbourne, et al., in “Chromatic Dispersion Compensation Device,” European Patent Application EP 1 098 211 A1, and/or for on-axis-illuminated multiple-coupled-cavity etalons in P. Colbourne, et al., in “Chromatic Dispersion Compensation Device,” U.S. patent application Ser. No. 2001/0021053 A1.